Cell adhesion is a process by which cells associate with each other, migrate towards a specific target, or localize within the extracellular matrix. Cell adhesion constitutes one of the fundamental mechanisms underlying numerous biological phenomena. For example, cell adhesion is responsible for the adhesion of hematopoietic cells to endothelial cells and the subsequent migration of those hematopoietic cells out of blood vessels and to the site of injury. As such, cell adhesion plays a role in pathologies such as inflammation, autoimmune disease, and tumor metastasis in mammals.
Investigations into the molecular basis for cell adhesion have revealed that various cell surface macromolecules, collectively known as cell adhesion molecules or receptors, mediate cell-cell and cell-matrix interactions. For example, members of the integrin family of cell surface receptors mediate cell-cell and cell-matrix interactions and regulate cell motility, migration, survival, and proliferation (Hynes, Cell, 69:11-25 (1992); Hynes, Cell, 110:673-687 (2002)). Integrins are non-covalent heterodimeric complexes consisting of two subunits, α and β. There are at least 18 different α subunits and at least 8 different β subunits.
Integrins are implicated in a variety of diseases and disorders, such as cancer, inflammation, autoimmune diseases, and genetic diseases. For example, α5β1, αvβ3, and αvβ5 integrins play critical roles in promoting tumor metastasis and angiogenesis (Hood and Cheresh, Nat. Rev. Cancer, 2:91-100 (2002); Jin and Varner, Brit. J Cancer, 90:561-565 (2004)). In addition, α4β1 integrin is involved in various developmental, physiological, and pathological processes.
α4β1 integrin, also known as very late antigen-4 (VLA-4) or CD49d/CD29, is a leukocyte cell surface receptor that participates in a wide variety of both cell-cell and cell-matrix adhesive interactions (Hemler, Ann. Rev. Immunol., 8:365 (1990)). α4β1 integrin is implicated in metastasis (Holzmann et al., Curr. Top. Microbio. Immunol., 231:125-141 (1998)), regulates leukocyte trafficking, and plays a critical role in inflammation and autoimmune diseases (Yusuf-Makagiansar et al., Med. Res. Reviews, 22:146-167 (2002)). For example, α4β1 integrin promotes tumor cell dissemination in distal organs by strengthening their adhesion to the vascular endothelium and facilitating their extravasation (Holzmann et al., id; Hauzenberger et al., Int. J. Cancer, 72:1034-1044 (1997)). In chronic lymphocytic leukemia (CLL), α4β1 integrin expression correlates with the presence of lymphadenopathy and determines the entry of the leukemia cells into nodes (Vincent et al., Blood, 87:4780-4788 (1996); Till et al., Blood, 15:2977-2984 (2002)).
Natural ligands for α4β1 integrin include vascular cell adhesion molecule-1 (VCAM-1) and fibronectin (FN). α4β1 integrin recognizes the primary amino acid sequence Gln-Ile-Asp-Ser (QIDS; SEQ ID NO:2) in VCAM-1 and Ile-Leu-Asp-Val (ILDV; SEQ ID NO:3) in FN. Blocking α4β1 interaction with its ligands has been used as a therapeutic strategy for inflammation and autoimmune diseases. For example, monoclonal antibodies to α4β1 integrin have been widely studied for their therapeutic effects. However, there are disadvantages using monoclonal antibody-based therapy due to factors such as low relative efficacy/safety ratios, especially in terms of systemic administration and immunogenic potential. To overcome these disadvantages, derivatives of the ILDV (SEQ ID NO:3)or QIDS (SEQ ID NO:2)sequence in the form of peptide, peptidomimetic, and small molecule non-peptide analogs are of particular interest (Helena et al., ibid).
By screening a random peptide library with an intact Jurkat T-leukemia cell line, the amino acid sequence Leu-Asp-Ile (LDI) was identified as a unique motif that binds preferentially to α4β1 integrin receptors on human lymphoid malignant cells and not to normal human peripheral lymphocytes (Park et al., Lett. Pept. Sci., 8:171-178 (2002)). The LDI peptide motif also binds preferentially to fresh leukemia cells isolated from patients with acute lymphocytic leukemia. As such, the activated form of α4β1 integrin is an attractive therapeutic or imaging target for human lymphoid malignancies, e.g., non-Hodgkins lymphoma, acute lymphocytic leukemia, and chronic lymphocytic leukemia, or for other cancers that over-express α4β1 integrin.
By using the ILDV (SEQ ID NO:3) sequence in FN as the starting point for inhibitor design, a series of α4β1 integrin inhibitors were developed (Chen et al., Biochem., 37:8743-8753 (1998)). One of the inhibitors, BIO-1211, was generated by substituting the Ile in ILDV with a 4-((N′-2-methylphenyl)ureido)-phenylacetyl N-terminal cap and adding a Pro (P) residue at the C-terminus. BIO-1211 is a potent α4β1 integrin inhibitor and selectively binds to the activated form of the receptor (Lin et al., J. Med. Chem., 42:920-934 (1999)). However, all of the α4β1 integrin inhibitors to date, including BIO-1211, have been designed as specific therapy for inflammatory and autoimmune diseases, and not for cancer. Further, these α4β1 integrin inhibitors suffer from the significant disadvantage of being susceptible to proteolysis by proteases found, for example, in plasma, the gastrointestinal tract, and tumor cells.
Thus, there is a need to develop α4β1 integrin inhibitors that (1) bind to α4β1 integrin with higher specificity and affinity than BIO-1211; (2) bind with high specificity and affinity to tumor cells (e.g., leukemia cells); and (3) are more resistant to cleavage or degradation from proteases found, for example, in plasma, the gastrointestinal tract, and tumor cells. The present invention satisfies this and other needs.